Study on control of axisymmetric jet by half delta-wing shaped vortex generators (Part 1: Evaluation of statistics on the velocity field)

Abstract

Effects of vortex generators (VGs) on velocity field in an axisymmetric jet are experimentally investigated through hotwire measurements. The VGs have a shape of a half delta-wing and they are installed around the jet exit. In addition to the case without the VGs insertion, three control modes of the VGs are tested: static mode, in which all the VGs are kept inserted, axisymmetric mode, in which all the VGs are inserted at an operation frequency of 3.0 Hz, and alternating mode, in which 3 adjacent VGs are inserted at an opposite phase. The jet Reynolds number is set to 20,000. The results show that the VGs promote jet diffusion and entrainment of the ambient fluid near the jet exit. However, this results in gentler velocity gradient in the radial direction, leading to smaller velocity fluctuation and Reynolds shear stress. Consequently, such promotion effect becomes weak as the jet proceeds in the downstream direction and the entrainment rate in the cases with the VGs insertion becomes smaller than that in the case without the VGs insertion. The active motion of the VGs does not contribute to jet control and the effectiveness of the VGs mainly depends on the insertion time in the present study because the frequency of the VGs operation and the shedding frequency of coherent eddies of the jet are different by two order of magnitude.